Let D be a set of n pairwise disjoint unit disks in the plane. We describe how to build a data structure for D so that for any point set P containing exactly one point from each disk, we can quickly nd the onion decomposition (convex layers) of P. Our ... read more data structure can be built in O(n log n) time and has linear size. Given P, we can nd its onion decomposition in O(n log k) time, where k is the number of layers. We also provide a matching lower bound. Our solution is based on a recursive space decomposition, combined with a fast algorithm to compute the union of two disjoint onion decompositions. show less